In the fabrication of modern semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of the devices. In recent years, a flip-chip attachment method or a flip-chip direct chip attachment (DCA) method has been used in packaging integrated circuit chips. In the flip-chip attachment method, instead of attaching an integrated circuit lead frame in a package, an array of solder balls is formed on the surface of the die for the subsequent bonding to a printed circuit board or an interposer. The formation of the solder balls can be carried out by an evaporation method utilizing a solder material consisting mainly of tin and lead through a mask to produce the balls in a desired pattern. More recently, the techniques of eletrodeposition or printing have been used to mass produce solder balls in a flip-chip packaging process.
In the flip-chip attachment method, all the interconnections between a semiconductor chip and a printed circuit board (PCB) or an interposer can be formed simultaneously and therefore maximizing fabrication throughputs. For instance, in direct chip attachment, solder bumps or solder balls are used to connect a chip directly to a printed circuit board or an interposer. In a regular printed circuit board, the density of the interconnections is not formed high enough to match that normally found on a chip surface. In other words, the pitch between the bond pads formed on a chip is smaller than the pitch formed between interconnections on a printed circuit board. An interposer is therefore used to provide a transition and to accommodate the bond pads/interconnections which are spaced differently. An interposer board is frequently fabricated of the same material as that used in the printed circuit board, i.e., an epoxy-type polymeric material. When a high density interconnect printed circuit board is utilized in a flip-chip method for bonding to a semiconductor chip, the use of the interposer may not be necessary.
The use of organic substrates, or polymeric-base substrates, in printed circuit boards or interposers introduces a new source of problem for the flip-chip bonding of a silicon chip which is mostly inorganic to such substrates. The problem is the mismatch of coefficients of thermal expansion (CTE) between the printed circuit board and the silicon chip. The coefficient of thermal expansion for the printed circuit board material is at least five times that of the silicon material. The extreme mismatch in CTE's between the silicon chip and the organic substrate of the printed circuit board therefore subjects solder joints formed thereinbetween to extremely large thermal strains, which leads to premature failure of the solder connections.
One method proposed for alleviating such thermal strains is the introduction of an encapsulating layer between the silicon chip and the organic substrate. The encapsulating material, known as an underfill, which is typically a silica filled epoxy is used to fill the gap (or standoff) between the printed circuit board and the silicon chip. Since the silicon chip is normally covered, in a final fabrication step, by a polymer passivation/stress buffer layer such as a polyimide film, the underfill forms a bond between the polyimide layer on the chip and the organic substrate of the printed circuit board encapsulating the solder joints.
Referring initially to FIG. 1, wherein a flip-chip 10 bonded by a plurality of solder balls 12 and an underfill layer 14 is shown. The encapsulating material, or the underfill layer 14, is typically a silica fill epoxy for filling the gap, or the standoff, between the printed circuit board 16 and the silicon chip 18. As shown in FIG. 1, the underfill layer 14 forms a bond between a polyimide layer 20, which is a passivation/stress buffer layer that covers the silicon chip 18, and the printed circuit board 16 encapsulating the solder balls 12.
While the introduction of the underfill layer between a silicon chip and an organic substrate for the printed circuit board has enhanced the thermal cycling resistance of a flip chip assembly, the dispensing of the underfill material in between a silicon chip and a substrate and filling the gap is a time consuming task. In one conventional method, as shown in FIGS. 2A.about.2C, an underfill dispenser 22 is first used to dispense an underfill material 24 onto the top surface 26 of a substrate 28. A layer 30 of the underfill material 24 is thus formed on the top surface 26. A chip holder 32, usually a vacuum holder, is then used to position an IC chip 34 which is pre-deposited with a plurality of solder balls 36 on a top surface 38 over the substrate 28. The IC chip, or die 34 is then pressed onto the substrate 28 with the plurality of solder balls 36 connecting to corresponding electrical conductors (not shown) on the surface 26 of the substrate 28. The assembly 40 for the flip chip is then placed in a reflow oven and heated to a temperature not less than the reflow temperature for the solder material utilized in the plurality of solder balls 36. The reflow process further cures the underfill material 30 and improves its mechanical strength.
Several drawbacks are inherent in this technique, for instance, there is possibly an underfill material layer between the plurality of solder balls on the IC chip and the plurality of electrical conductors on the substrate. Since the underfill material is an insulating material, this affects the contact resistance formed between the joints. Secondly, in the process of pressing the IC die 34 onto the underfill material layer 30, air entrapment in the underfill material 30 is inevitable. Trapped air bubbles in the underfill material layer 30, or in the epoxy material layer 30, affects the mechanical strength enhancement by the underfill material and furthermore, affects the adhesion formed between the underfill material and the IC die or the substrate.
In another conventional technique for dispensing underfill materials, as shown in FIGS. 3A.about.3F, an underfill material is fed into the standoff between an IC die and a substrate by the capillary effect on the underfill liquid. As shown in FIG. 3A, a wafer 42 is first sectioned into individual dies 44 by a diamond saw 46. The IC dies 44 are provided with a plurality of solder balls 48 on a top surface 50 of the dies. After all the dies 44 are severed from wafer 42, they are placed in a holding tray 52, as shown in FIG. 3B. In the next step of the process, a vacuum head 54 is used to remove an IC die 44 from the tray 52 and to position the die over a substrate 56. It is noted that a plurality of electrical conductors 58, corresponding to the number and positions of the solder balls 48 are provided on a top surface 60 of the substrate 56. It should be noted that the substrate 56 may be either a printed circuit board or an interposer. After the IC die 44 is mounted to substrate 56 by intimately contacting the solder balls 48 with the electrical conductors 58, as shown in FIG. 3D, a solder reflow process is carried out to reflow the solder and to form a permanent bond between the IC die 44 and the substrate 56. It should be noted that the plurality of electrical conductors 58 are not shown in FIG. 3D for simplicity reasons.
The flip chip package 62 is now ready for the underfill process in which an underfill dispenser 64, such as a liquid syringe, is used to dispense an underfill material 66 at an edge of the flip chip package 62. Since a gap 68, or the standoff, between the chip 44 and the substrate 56 is relatively small, i.e., in the neighborhood between about 50 .mu.m and about 100 .mu.m, a capillary effect causes the underfill material 66 to flow into the gap 68 and fill up the gap. Since the underfill dispensing process utilizes capillary effect, several factors may influence the underfill filling process. For instance, the viscosity of the underfill materials 56 and the temperature of the substrate 56 and the IC die 44. Moreover, the capillary flow process for the underfill material 56 is time consuming, i.e., up to 1 minute flow time is required to fill under an IC die which has a dimension of 10 mm.times.10 mm. A completed flip chip 62 with the underfill dispensed between the IC die 44 and the substrate 56 is shown in FIG. 3F.
It is therefore an object of the present invention to provide a method for dispensing underfill in a flip chip package that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for dispensing underfill in a flip chip package that does not require a long flow time for the underfill material to fill a gap between an IC die and a substrate.
It is a further object of the present invention to provide a method for dispensing an underfill material in a flip chip package that utilizes a screen printing or a stencil printing technique for achieving a short cycle time.
It is still another object of the present invention to provide a method for dispensing an underfill material in a flip chip package that utilizes a screen printing or a stencil printing technique such that a multiple number of flip chip packages may be fabricated at the same time.
It is another further object of the present invention to provide a method for dispensing an underfill material in a flip chip package that utilizes a dispensing nozzle head equipped with a plurality of liquid dispensing nozzles such that droplets of underfill material may be dispensed on top of an IC die filling the gaps between solder balls.
It is yet another object of the present invention to provide a method for dispensing an underfill in a flip chip package that utilizes a dispensing nozzle head equipped with a plurality of liquid dispensing nozzles such that droplets of an underfill material may be dispensed onto the surface of a substrate and thus filling the gaps between electrical conductors.
It is still another further object of the present invention to provide a method for dispensing underfill in a flip chip package that is formed of an IC chip and a substrate by utilizing a stencil equipped with slot openings in a tapered form for dispensing an underfill material by a squeegee into the standoff between the chip and the substrate.
It is yet another further object of the present invention to provide a method for dispensing underfill in a flip chip package formed of an IC chip and a substrate in which the solder reflow process and the underfill curing process are conducted in the same heat treatment process.